The two packages and other suspicious parcels aboard cargo jets, all originating from Yemen, 'underscore the necessity of remaining vigilant against terrorism,' President Obama says.

A Jewish Federation of Greater Chicago spokeswoman said the group was "taking appropriate precautions" and was "advising our local synagogues to do likewise." One of the targets was a Jewish congregation that meets at a Unitarian church, according to a U.S. official.

Oren Segal, director of the Anti-Defamation League's Center on Extremism in Washington, said Rahm Emanuel has been the focus of some attention on extremist blogs since long before he resigned as White House chief of staff to run for Chicago mayor. Segal said that vitriol on message boards peaked when Obama named Emanuel his top aide in early 2009.

The two incidents highlight a known vulnerability in the air cargo industry, one that has been the subject of extensive discussion between the Transportation Security Administration and the industry for several years.

The federal government has mandated in recent years that all cargo on passenger aircraft be screened, a goal that was achieved only this August. But the issue of parcels aboard cargo-only aircraft has been far more difficult to resolve. As far back as March 2009, the industry warned Congress it would not be able to meet the August deadline that 100% of cargo would be screened.

A TSA official acknowledged Friday that not all cargo inbound from abroad is screened and that the cargo that does get screened is handled differently than passenger luggage, which is subject to X-ray. That means that the two suspicious packages may not have been subject to screening when they were originally loaded in Yemen.